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Gamma-ray bursts (GRBs) are the most electromagnetically luminous cosmic explosions. They are powered by collimated streams of plasma (jets) ejected by a newborn stellar-mass black hole or neutron star at relativistic velocities (near the speed of light). Their short-lived (typically tens of seconds) prompt $\gamma$-ray emission from within the ejecta is followed by long-lived multi-wavelength afterglow emission from the ultra-relativistic forward shock. This shock is driven into the circumburst medium by the GRB ejecta that are in turn decelerated by a mildly-relativistic reverse shock. Forward shock emission was recently detected up to teraelectronvolt-energy $\gamma$-rays, and such very-high-energy emission was also predicted from the reverse shock. Here we report the detection of optical and gigaelectronvolt-energy $\gamma$-ray emission from GRB 180720B during the first few hundred seconds, which is explained by synchrotron and inverse-Compton emission from the reverse shock propagating into the ejecta, implying a low-magnetization ejecta. Our optical measurements show a clear transition from the reverse shock to the forward shock driven into the circumburst medium, accompanied by a 90-degree change in the mean polarization angle and fluctuations in the polarization degree and angle. This indicates turbulence with large-scale toroidal and radially-stretched magnetic field structures in the reverse and forward shocks, respectively, which tightly couple to the physics of relativistic shocks and GRB jets -- launching, composition, dissipation and particle acceleration., Comment: 5 pages, 4 figures (main) plus Methods and Supplementary Methods, accepted for publication

Time-resolved linear polarization ($\Pi$) measurements of the prompt gamma-ray burst emission can reveal its dominant radiation mechanism. A widely considered mechanism is synchrotron radiation, for which linear polarization can be used to probe the jet's magnetic-field structure, and in turn its composition. In axisymmetric jet models the polarization angle (PA) can only change by $90^\circ$, as $\Pi$ temporarily vanishes. However, some time-resolved measurements find a continuously changing PA, which requires the flow to be non-axisymmetric in at least one out of its emissivity, bulk Lorentz factor or magnetic field. Here we consider synchrotron emission in non-axisymmetric jets, from an ultrarelativistic thin shell, comprising multiple radially-expanding mini-jets (MJs) or emissivity patches within the global jet, that yield a continuously changing PA. We explore a wide variety of possibilities with emission consisting of a single pulse or multiple overlapping pulses, presenting time-resolved and integrated polarization from different magnetic field configurations and jet angular structures. We find that emission from multiple incoherent MJs/patches reduces the net polarization due to partial cancellation in the Stokes plane. When these contain a large-scale ordered field in the plane transverse to the radial direction, $\Pi$ always starts near maximal and then declines over the single pulse or shows multiple highly polarized peaks due to multiple pulses. Observing $\Pi\lesssim40\%$ ($15\%$) integrated over one (several) pulse(s) will instead favor a shock-produced small-scale field either ordered in the radial direction or tangled in the plane transverse to it., Comment: 18 pages, 20 figures; Added table 1 and section 6; Accepted for publication in MNRAS

Exceptionally bright gamma-ray burst (GRB) afterglows can reveal the angular structure of their jets. GRB jets appear to have a narrow core (of half-opening angle $\theta_c$), beyond which their kinetic energy drops as a power-law with angle $\theta$ from the jet's symmetry axis, $E_{k,\rm iso}(\theta)\propto[1+(\theta/\theta_c)^2]^{-a/2}$. The power-law index $a$ reflects the amount of mixing between the shocked jet and confining medium, which depends on the jet's initial magnetization. Weakly magnetized jets undergo significant mixing, leading to shallow ($a\lesssim2$) angular profiles. We use the exquisite multi-waveband afterglow observations of GRB 221009A to constrain the jet angular structure using a dynamical model that accounts for both the forward and reverse shocks, for a power-law external density profile, $n_{\rm{}ext}\propto{}R^{-k}$. Both the forward-shock emission, that dominates the optical and X-ray flux, and the reverse-shock emission, that produces the radio afterglow, require a jet with a narrow core ($\theta_c\approx0.021$) and a shallow angular structure ($a\approx0.8$) expanding into a stellar wind ($k\approx2$). Moreover, these data appear to favor a small fraction ($\xi_e\approx10^{-2}$) of shock-heated electrons forming a power-law energy distribution in both shocks., Comment: 5 pages, 3 figures. Accepted to MNRAS Letters

The ultra-relativistic outflows powering gamma-ray bursts (GRBs) acquire angular structure through their interaction with stellar material (dynamical ejecta) in long-soft (short-hard) GRBs. They can often be characterized by a compact, nearly uniform narrow core (with half-opening angle $\theta_{c,\{\epsilon,\Gamma\}}$) surrounded by material with energy per unit solid angle ($\epsilon=\epsilon_c\Theta_{\epsilon}^{-a}$, where $\Theta_{\{\epsilon,\Gamma\}}=[1+\theta^2/\theta_{c,\{\epsilon,\Gamma\}}^2]^{1/2}$) and initial specific kinetic energy ($\Gamma_0-1=[\Gamma_c-1]\Theta_\Gamma^{-b}$) declining as power laws. Multi-wavelength afterglow lightcurves of off-axis jets (with viewing angle $\theta_{obs} >\theta_c$) offer robust ways to constrain a, b and the external density radial profile ($\rho\propto R^{-k}$), even while other burst parameters may remain highly degenerate. We extend our work BGG2020 on such afterglows to include more realistic angular structure profiles derived from three-dimensional hydrodynamic simulations of GRBs (addressing also jets with shallow angular energy profiles, whose emission exhibits unique evolution). We present afterglow lightcurves based on our parameterized power-law jet angular profiles for different viewing angles $\theta_{obs}$ and k=0,1,2. We identify a unique evolutionary power-law phase of the characteristic synchrotron frequencies ($\nu_m$ and $\nu_c$) that manifests when the lightcurve is dominated by emission sensitive to the angular structure of the outflow. We calculate the criterion for obtaining single or double peaked light curves in the general case when $\theta_{c,\Gamma}\neq\theta_{c,\epsilon}$. Following our earlier work, we emphasize how the shape of the lightcurve and the temporal evolution of $\nu_m$ and $\nu_c$ can be used to constrain the outflow structure and potentially distinguish between magnetic and hydrodynamic jets., Comment: 15 pages, 13 figures. Submitted

We present numerical simulation results for the propagation of Alfv\'{e}n waves in the charge starvation regime. This is the regime where the plasma density is below the critical value required to supply the current for the wave. We analyze a conservative scenario where Alfv\'{e}n waves pick up charges from the region where the charge density exceeds the critical value and advect them along at a high Lorentz factor. The system consisting of the Alfv\'{e}n wave and charges being carried with it, which we call charge-carrying Alfv\'{e}n wave (CC-AW), moves through a medium with small, but non-zero, plasma density. We find that the interaction between CC-AW and the stationary medium has a 2-stream like instability which leads to the emergence of a strong electric field along the direction of the unperturbed magnetic field. The growth rate of this instability is of order the plasma frequency of the medium encountered by the CC-AW. Our numerical code follows the system for hundreds of wave periods. The numerical calculations suggest that the final strength of the electric field is of order a few percent of the Alfv\'{e}n wave amplitude. Little radiation is produced by the sinusoidally oscillating currents associated with the instability during the linear growth phase. However, in the nonlinear phase, the fluctuating current density produces strong EM radiation near the plasma frequency and limits the growth of the instability., Comment: 12 pages, 6 figures, accepted for publication in MNRAS

Gamma-ray bursts (GRBs) are the most luminous explosions in the Universe and are powered by ultra-relativistic jets. Their prompt $\gamma$-ray emission briefly outshines the rest of the $\gamma$-ray sky making them detectable from cosmological distances. It is followed by, and sometimes partially overlaps with, a similarly energetic but very broadband and longer-lasting afterglow emission. While most GRBs are detected below a few MeV, over a hundred were detected at high ($\gtrsim0.1\;$GeV) energies and several have now been observed up to tens of GeV with the \textit{Fermi} Large Area Telescope (LAT). A new electromagnetic window in the very high energy (VHE) domain ($\gtrsim0.1\;$TeV) was recently opened with the detection of afterglow emission in the $(0.1$\textendash$1)\,$TeV energy band by ground-based imaging atmospheric Cherenkov telescopes. The emission mechanism for the VHE spectral component is not fully understood, and its detection offers important constraints for GRB physics. This review provides a brief overview of the different leptonic and hadronic mechanisms capable of producing VHE emission in GRBs. The same mechanisms possibly give rise to the high-energy spectral component seen during the prompt emission of many \textit{Fermi}-LAT GRBs. Possible origins of its delayed onset and long duration, well into the afterglow phase, with implications for the emission region and relativistic collisionless shock physics are discussed. Key results for using GRBs as ideal probes for constraining models of extra-galactic background light and intergalactic magnetic fields, as well as for testing Lorentz invariance violation, are presented., Comment: 29 pages, 7 figures, invited review for the journal Galaxies as part of the special issue on Extragalactic TeV Astronomy. Accepted for publication

The ultra-relativistic outflows powering gamma-ray bursts (GRBs) acquire angular structure through their interaction with external material. They are often characterized by a compact, nearly uniform narrow core (with half-opening angle $\theta_{c,\{\epsilon,\Gamma\}}$) surrounded by material with energy per unit solid angle ($\epsilon=\epsilon_c\Theta_{\epsilon}^{-a}$, where $\Theta_{\{\epsilon,\Gamma\}}=[1+\theta^2/\theta_{c,\{\epsilon,\Gamma\}}^2]^{1/2}$) and initial specific kinetic energy ($\Gamma_0-1=[\Gamma_c-1]\Theta_\Gamma^{-b}$) declining as power laws. Multi-wavelength afterglow lightcurves of off-axis jets (with viewing angle $\theta_{\rm obs} > \theta_c$) offer robust ways to constrain $a$, $b$ and the external density radial profile ($\rho\propto R^{-k}$), even while other burst parameters may remain highly degenerate. We extend our previous work on such afterglows to include more realistic angular structure profiles derived from three-dimensional hydrodynamic simulations of both long and short GRBs (addressing also jets with shallow angular energy profiles, whose emission exhibits unique evolution). We present afterglow lightcurves based on our parameterized power-law jet angular profiles for different viewing angles $\theta_{\rm obs}$ and $k=\{0,1,2\}$. We identify a unique evolutionary power-law phase of the characteristic synchrotron frequencies ($\nu_m$ and $\nu_c$) that manifests when the lightcurve is dominated by emission sensitive to the angular structure of the outflow. We calculate the criterion for obtaining single or double peaked light-curves in the general case when $\theta_{c,\Gamma}\neq\theta_{c,\epsilon}$. We emphasize how the shape of the lightcurve and the temporal evolution of $\nu_m$ and $\nu_c$ can be used to constrain the outflow structure and potentially distinguish between magnetic and hydrodynamic jets., Comment: 15 pages, 13 figures. Accepted for publication in MNRAS

The origin of the variable X-ray emission in the $(0.3-30)\,$keV energy range of ultraluminous X-ray sources (ULXs) remains unclear, making it difficult to constrain the mass of the central compact object. X-ray luminosities of bright ULXs can be explained with sub-critical accretion ($L

Over half a century from the discovery of gamma-ray bursts (GRBs), the dominant radiation mechanism responsible for their bright and highly variable prompt emission remains poorly understood. Spectral information alone has proven insufficient for understanding the composition and main energy dissipation mechanism in GRB jets. High-sensitivity polarimetric observations from upcoming instruments in this decade may help answer such key questions in GRB physics. This article reviews the current status of prompt GRB polarization measurements and provides comprehensive predictions from theoretical models. A concise overview of the fundamental questions in prompt GRB physics is provided. Important developments in gamma-ray polarimetry including a critical overview of different past instruments are presented. Theoretical predictions for different radiation mechanisms and jet structures are confronted with time-integrated and time-resolved measurements. The current status and capabilities of upcoming instruments regarding the prompt emission are presented. The very complimentary information that can be obtained from polarimetry of X-ray flares as well as reverse-shock and early to late forward-shock (afterglow) emission is highlighted. Finally, promising directions for overcoming the inherent difficulties in obtaining statistically significant prompt-GRB polarization measurements are discussed, along with prospects for improvements in the theoretical modeling, which may lead to significant advances in the field., Comment: 78 pages, 19 figures; minor text edits; updated Fig. 14; invited review article accepted to Galaxies special issue 'Gamma-Ray Burst Science in 2030'

The Gamow Explorer will use Gamma Ray Bursts (GRBs) to: 1) probe the high redshift universe (z < 6) when the first stars were born, galaxies formed and Hydrogen was reionized; and 2) enable multi-messenger astrophysics by rapidly identifying Electro-Magnetic (IR/Optical/X-ray) counterparts to Gravitational Wave (GW) events. GRBs have been detected out to z ~ 9 and their afterglows are a bright beacon lasting a few days that can be used to observe the spectral fingerprints of the host galaxy and intergalactic medium to map the period of reionization and early metal enrichment. Gamow Explorer is optimized to quickly identify high-z events to trigger follow-up observations with JWST and large ground-based telescopes. A wide field of view Lobster Eye X-ray Telescope (LEXT) will search for GRBs and locate them with arc-minute precision. When a GRB is detected, the rapidly slewing spacecraft will point the 5 photometric channel Photo-z Infra-Red Telescope (PIRT) to identify high redshift (z < 6) long GRBs within 100s and send an alert within 1000s of the GRB trigger. An L2 orbit provides < 95% observing efficiency with pointing optimized for follow up by the James Webb Space Telescope (JWST) and ground observatories. The predicted Gamow Explorer high-z rate is <10 times that of the Neil Gehrels Swift Observatory. The instrument and mission capabilities also enable rapid identification of short GRBs and their afterglows associated with GW events. The Gamow Explorer will be proposed to the 2021 NASA MIDEX call and if approved, launched in 2028.

The origin of the variable X-ray emission in the $(0.3-30)\,$keV energy range of ultraluminous X-ray sources (ULXs) remains unclear, making it difficult to constrain the mass of the central compact object. X-ray luminosities of bright ULXs can be explained with sub-critical accretion ($L

Over half a century from the discovery of gamma-ray bursts (GRBs), the dominant radiation mechanism responsible for their bright and highly variable prompt emission remains poorly understood. Spectral information alone has proven insufficient for understanding the composition and main energy dissipation mechanism in GRB jets. High-sensitivity polarimetric observations from upcoming instruments in this decade may help answer such key questions in GRB physics. This article reviews the current status of prompt GRB polarization measurements and provides comprehensive predictions from theoretical models. A concise overview of the fundamental questions in prompt GRB physics is provided. Important developments in gamma-ray polarimetry including a critical overview of different past instruments are presented. Theoretical predictions for different radiation mechanisms and jet structures are confronted with time-integrated and time-resolved measurements. The current status and capabilities of upcoming instruments regarding the prompt emission are presented. The very complimentary information that can be obtained from polarimetry of X-ray flares as well as reverse-shock and early to late forward-shock (afterglow) emission is highlighted. Finally, promising directions for overcoming the inherent difficulties in obtaining statistically significant prompt-GRB polarization measurements are discussed, along with prospects for improvements in the theoretical modeling, which may lead to significant advances in the field., Comment: 78 pages, 19 figures; minor text edits; updated Fig. 14; invited review article accepted to Galaxies special issue 'Gamma-Ray Burst Science in 2030'

The dominant radiation mechanism that produces the prompt emission in gamma-ray bursts (GRBs) remains a major open question. Spectral information alone has proven insufficient in elucidating its nature. Time-resolved linear polarization has the potential to distinguish between popular emission mechanisms, e.g., synchrotron radiation from electrons with a power-law energy distribution or inverse Compton scattering of soft seed thermal photons, which can yield the typical GRB spectrum but produce different levels of polarization. Furthermore, it can be used to learn about the outflow's composition (i.e. whether it is kinetic-energy-dominated or Poynting-flux-dominated) and angular structure. For synchrotron emission it is a powerful probe of the magnetic field geometry. Here we consider synchrotron emission from a thin ultrarelativistic outflow, with bulk Lorentz factor $\Gamma(R)=\Gamma_0(R/R_0)^{-m/2}\gg1$, that radiates a Band-function spectrum in a single (multiple) pulse(s) over a range of radii, $R_0\leq R\leq R_0+\Delta R$. Pulse profiles and polarization evolution at a given energy are presented for a coasting ($m=0$) and accelerating ($m=-2/3$) thin spherical shell and for an off-axis top-hat jet with sharp as well as smooth edges in emissivity. Four different magnetic field configurations are considered, such as a locally ordered field coherent over angular scales $\theta_B\gtrsim1/\Gamma$, a tangled field ($B_\perp$) in the plane transverse to the radial direction, an ordered field ($B_\parallel$) aligned in the radial direction, and a globally ordered toroidal field ($B_{\rm tor}$). All field configurations produce distinct polarization evolution with single (for $B_\perp$ and $B_\parallel$) and double (for $B_{\rm tor}$) $90^\circ$ changes in the polarization position angle., Comment: 20 pages, 11 figures; added two more panels to Fig. 6 and main points of paper to the summary section; Accepted to MNRAS

The observations of GW170817/GRB170817A have confirmed that the coalescence of a neutron-star binary is the progenitor of a short gamma-ray burst. In the standard picture of a short gamma-ray burst, a collimated highly relativistic outflow is launched after merger and it successfully breaks out from the surrounding ejected matter. Using initial conditions inspired from numerical-relativity binary neutron-star merger simulations, we have performed general-relativistic hydrodynamic (HD) and magnetohydrodynamic (MHD) simulations in which the jet is launched and propagates self-consistently. The complete set of simulations suggests that: (i) MHD jets have an intrinsic energy and velocity polar structure with a ``hollow core'' subtending an angle $\theta_{\rm core}\approx4^{\circ}-5^{\circ}$ and an opening angle of $\theta_{\rm jet}\gtrsim10^{\circ}$; (ii) MHD jets eject significant amounts of matter and two orders of magnitude more than HD jets; (iii) the energy stratification in MHD jets naturally yields the power-law energy scaling $E(>\Gamma\beta)\propto(\Gamma\beta)^{-4.5}$; (iv) MHD jets provide fits to the afterglow data from GRB170817A that are comparatively better than those of the HD jets and without free parameters; (v) finally, both of the best-fit HD/MHD models suggest an observation angle $\theta_{\rm obs} \simeq 21^{\circ}$ for GRB170817A., Comment: 9 pages, 5 figures, submitted

GRB 170817A/GW 170817 is the first GRB clearly viewed far from the GRB jet's symmetry axis. Its afterglow was densely monitored over a wide range of frequencies and times. It has been modeled extensively, primarily numerically, and although this endeavour was very fruitful, many of the underlying model parameters remain undetermined. We provide analytic modelling of GRB afterglows observed off-axis, considering jets with a narrow core (of half-opening angle $\theta_{\rm c}$) and power-law wings in energy per unit solid angle ($\epsilon=\epsilon_c\Theta^{-a}$ where $\Theta=[1+(\theta/\theta_{\rm c})^2]^{1/2}$) and initial specific kinetic energy ($\Gamma_0-1=[\Gamma_{\rm c,0}-1]\Theta^{-b}$), as well as briefly discuss Gaussian jets. Our study reveals qualitatively different types of lightcurves that can be viewed in future off-axis GRBs, with either single or double peaks, depending on the jet structure and the viewing angle. Considering the lightcurve shape rather than the absolute normalizations of times and / or fluxes, removes the dependence of the lightcurve on many of the highly degenerate burst parameters. This study can be easily used to determine the underlying jet structure, significantly reduce the effective parameter space for numerical fitting attempts and provide physical insights. As an illustration, we show that for GRB 170817A, there is a strong correlation between the allowed values of $\Gamma_{\rm c,0}$ and $b$, leading to a narrow strip of allowed solutions in the $\Gamma_{\rm c,0}$-$b$ plane above some minimal values $\Gamma_{\rm c,0}\gtrsim 40, b\gtrsim1.2$. Furthermore, the Lorentz factor of the material dominating the early lightcurve can be constrained by three independent techniques to be $\Gamma_{0}(\theta_{\rm min,0})\approx5-7$., Comment: 14 pages, 17 figures. Published in MNRAS

We perform three-dimensional (3D) general-relativistic magnetohydrodynamic simulations to model the jet break-out from the ejecta expected to be produced in a binary neutron-star merger. The structure of the relativistic outflow from the 3D simulation confirms our previous results from 2D simulations, namely, that a relativistic magnetized outflow breaking out from the merger ejecta exhibits a hollow core of $\theta_{\rm core}\approx4^{\circ}$, an opening angle of $\theta_{\rm jet}\gtrsim10^{\circ}$, and is accompanied by a wind of ejected matter that will contribute to the kilonova emission. We also compute the non-thermal afterglow emission of the relativistic outflow and fit it to the panchromatic afterglow from GRB170817A, together with the superluminal motion reported from VLBI observations. In this way, we deduce an observer angle of $\theta_{\rm obs}= 35.7^{\circ \,\,+1.8}_{\phantom{\circ \,\,}-2.2}$. We further compute the afterglow emission from the ejected matter and constrain the parameter space for a scenario in which the matter responsible for the thermal kilonova emission will also lead to a non-thermal emission yet to be observed., Comment: MNRAS accepted, updated version

Modeling of gamma-ray burst (GRB) prompt emission spectra sometimes requires a (quasi-) thermal spectral component in addition to the Band function. In photospheric emission models, a prominent thermal component broadened by sub-photospheric dissipation is expected to be released at the photospheric radius, $r_{\rm ph}\sim10^{12}\,$cm. We consider an ultra-relativistic strongly magnetized outflow with a striped-wind magnetic-field structure undergoing gradual and continuous magnetic energy dissipation at $r

The observations of GW170817/GRB170817A have confirmed that the coalescence of a neutron-star binary is the progenitor of a short gamma-ray burst. In the standard picture of a short gamma-ray burst, a collimated highly relativistic outflow is launched after merger and it successfully breaks out from the surrounding ejected matter. Using initial conditions inspired from numerical-relativity binary neutron-star merger simulations, we have performed general-relativistic hydrodynamic (HD) and magnetohydrodynamic (MHD) simulations in which the jet is launched and propagates self-consistently. The complete set of simulations suggests that: (i) MHD jets have an intrinsic energy and velocity polar structure with a ``hollow core'' subtending an angle $\theta_{\rm core}\approx4^{\circ}-5^{\circ}$ and an opening angle of $\theta_{\rm jet}\gtrsim10^{\circ}$; (ii) MHD jets eject significant amounts of matter and two orders of magnitude more than HD jets; (iii) the energy stratification in MHD jets naturally yields the power-law energy scaling $E(>\Gamma\beta)\propto(\Gamma\beta)^{-4.5}$; (iv) MHD jets provide fits to the afterglow data from GRB170817A that are comparatively better than those of the HD jets and without free parameters; (v) finally, both of the best-fit HD/MHD models suggest an observation angle $\theta_{\rm obs} \simeq 21^{\circ}$ for GRB170817A., Comment: 9 pages, 5 figures, submitted

GRB 170817A/GW 170817 is the first GRB clearly viewed far from the GRB jet's symmetry axis. Its afterglow was densely monitored over a wide range of frequencies and times. It has been modeled extensively, primarily numerically, and although this endeavour was very fruitful, many of the underlying model parameters remain undetermined. We provide analytic modelling of GRB afterglows observed off-axis, considering jets with a narrow core (of half-opening angle $\theta_{\rm c}$) and power-law wings in energy per unit solid angle ($\epsilon=\epsilon_c\Theta^{-a}$ where $\Theta=[1+(\theta/\theta_{\rm c})^2]^{1/2}$) and initial specific kinetic energy ($\Gamma_0-1=[\Gamma_{\rm c,0}-1]\Theta^{-b}$), as well as briefly discuss Gaussian jets. Our study reveals qualitatively different types of lightcurves that can be viewed in future off-axis GRBs, with either single or double peaks, depending on the jet structure and the viewing angle. Considering the lightcurve shape rather than the absolute normalizations of times and / or fluxes, removes the dependence of the lightcurve on many of the highly degenerate burst parameters. This study can be easily used to determine the underlying jet structure, significantly reduce the effective parameter space for numerical fitting attempts and provide physical insights. As an illustration, we show that for GRB 170817A, there is a strong correlation between the allowed values of $\Gamma_{\rm c,0}$ and $b$, leading to a narrow strip of allowed solutions in the $\Gamma_{\rm c,0}$-$b$ plane above some minimal values $\Gamma_{\rm c,0}\gtrsim 40, b\gtrsim1.2$. Furthermore, the Lorentz factor of the material dominating the early lightcurve can be constrained by three independent techniques to be $\Gamma_{0}(\theta_{\rm min,0})\approx5-7$., Comment: 14 pages, 17 figures. Published in MNRAS

Gamma-ray burst afterglows arise from relativistic collisionless shocks in which the postshock tangled magnetic field $\vec{B}$ is produced by the two-stream and/or Weibel instabilities on plasma skin-depth scales $(c/\omega_p)$. The field is expected to be oriented predominantly within the shock plane ($B_{\perp}$; transverse to the shock normal, $\hat{n}_{\rm{sh}}$), and is often approximated to be completely within it ($B_\parallel\equiv\hat{n}_{\rm{sh}}\,\cdot\,\vec{B}=0$). Current 2D/3D particle-in-cell simulations are limited to short timescales and box sizes $\lesssim10^4(c/\omega_p)\ll R/\Gamma_{\rm{sh}}$ much smaller than the shocked region's comoving width, and cannot probe the asymptotic downstream $\vec{B}$ structure. We constrain the latter using the linear polarization upper limit, $|\Pi|<12\%$, on the radio afterglow of GW170817/GRB170817A. Afterglow polarization depends on the jet's angular structure, our viewing angle, and the $\vec{B}$ structure. In GW170817/GRB170817A the latter can be tightly constrained since the former two are constrained from observations. We model $\vec{B}$ as an isotropic field in 3D that is stretched along $\hat{n}_{\rm{sh}}$ by a factor $\xi\equiv{}B_{\parallel}/B_{\perp}$, whose initial value $\xi_f\equiv{}B_{\parallel,f}/B_{\perp,f}$ describes the field that survives downstream on plasma scales $\ll{}R/\Gamma_{\rm{sh}}$. We calculate $\Pi(\xi_f)$ by integrating over the shocked volume for core-dominated structured jets, with a local Blandford-McKee self-similar radial profile. We find that independent of the exact jet structure, $\vec{B}$ has a finite, but initially sub-dominant, parallel component: $0.57\lesssim\xi_f\lesssim0.89$, making it less anisotropic. While this motivates numerical studies of the asymptotic $\vec{B}$ structure in relativistic collisionless shocks, it may be consistent with turbulence amplified magnetic field., Comment: 11 pages, 6 figures, Accepted to MNRAS

The afterglow of GRB$\,$170817A/GW$\,$170817 was very unusual, slowly rising as $F_\nu\propto{}t_{\rm{}obs}^{0.8}\nu^{-0.6}$, peaking at $t_{\rm{obs,pk}}\sim\,150\;$days, and sharply decaying as $\sim{}t_{\rm{}obs}^{-2.2}$. VLBI observations revealed an unresolved radio afterglow image whose flux centroid moved superluminally with $v_{\rm{app}}\approx4c$, clearly indicating that the afterglow was dominated by a relativistic jet's compact core. Different jet angular structures explained the afterglow lightcurves: Gaussian and steep power-law profiles with narrow core angles $\theta_c\lesssim5^\circ$ and larger viewing angles $\theta_{\rm{}obs}/\theta_c\sim3-5$. However, a top-hat jet (with sharp edges at $\theta=\theta_0$) was ruled out since it appeared to produce an early flux rise much steeper than observed. Using 2D relativistic hydrodynamic simulations we show that the initial steep flux rise is an artifact caused by the simulation's finite start time, $t_0$, missing its flux contributions from $t

We present ALMA 97.5 GHz total intensity and linear polarization observations of the mm-band afterglow of GRB 190114C spanning 2.2 to 5.2 hours after the burst. We detect linear polarization at the $\approx 5\,\sigma$ level, decreasing from $\Pi=(0.87\pm0.13)\%$ to $(0.60\pm0.19)\%$, and evolving in polarization position angle from $(10\pm5)^\circ$ to $(-44\pm12)^\circ$ during the course of the observations. This represents the first detection of polarized millimeter emission in a $\gamma$-ray burst. We show that the optical and X-ray observations between $0.03$ days and $\sim0.3$ days are consistent with a fast cooling forward shock expanding into a wind environment. However, the optical observations at $\lesssim0.03$ days, as well as the radio and millimeter observations arise from a separate component, which we interpret as emission from the reverse-shocked ejecta. Using the measured linear polarization, we constrain the coherence scale of tangled magnetic fields in the ejecta to an angular size of $\theta_{\rm B} \approx10^{-3}$ radian, while the rotation of the polarization angle rules out the presence of large scale, ordered axisymmetric magnetic fields, and in particular a large scale toroidal field, in the jet., Comment: Published in ApJ Letters

The main hard pulse of prompt gamma-ray emission in GRB$\,$170817A had a duration of $\sim0.5\,{\rm s}$ and its onset was delayed with respect to the gravitational-wave chirp signal by $t_{\rm del} \approx 1.74\,{\rm s}$. Detailed follow-up of the subsequent broadband kilonova emission revealed a two-component ejecta -- a lanthanide-poor ejecta with mass $M_{\rm ej,blue}\approx0.025\,M_\odot$ that powered the early but rapidly fading blue emission and a lanthanide-rich ejecta with mass $M_{\rm ej,red}\approx 0.04\,M_\odot$ that powered the longer lasting redder emission. Both the prompt gamma-ray onset delay and the existence of the blue ejecta with modest electron fraction, $0.2\lesssim Y_e\lesssim0.3$, can be explained if the collapse to a black hole was delayed by the formation of a hypermassive neutron star (HMNS). Here, we determine the survival time of the merger remnant by combining two different constraints, namely, the time needed to produce the requisite blue-ejecta mass and that necessary for the relativistic jet to bore its way out of the expanding ejecta. In this way, we determine that the remnant of GW170817 must have collapsed to a black hole after $t_{\rm coll}=0.98_{-0.26}^{+0.31}\,{\rm s}$. We also discuss how future detections and the delays between the gravitational and electromagnetic emissions can be used to constrain the properties of the merged object., Comment: 20 pages, 7 figures. Incorporated comments from the referee. Accepted for publication in ApJ

We present ALMA 97.5 GHz total intensity and linear polarization observations of the mm-band afterglow of GRB 190114C spanning 2.2 to 5.2 hours after the burst. We detect linear polarization at the $\approx 5\,\sigma$ level, decreasing from $\Pi=(0.87\pm0.13)\%$ to $(0.60\pm0.19)\%$, and evolving in polarization position angle from $(10\pm5)^\circ$ to $(-44\pm12)^\circ$ during the course of the observations. This represents the first detection of polarized millimeter emission in a $\gamma$-ray burst. We show that the optical and X-ray observations between $0.03$ days and $\sim0.3$ days are consistent with a fast cooling forward shock expanding into a wind environment. However, the optical observations at $\lesssim0.03$ days, as well as the radio and millimeter observations arise from a separate component, which we interpret as emission from the reverse-shocked ejecta. Using the measured linear polarization, we constrain the coherence scale of tangled magnetic fields in the ejecta to an angular size of $\theta_{\rm B} \approx10^{-3}$ radian, while the rotation of the polarization angle rules out the presence of large scale, ordered axisymmetric magnetic fields, and in particular a large scale toroidal field, in the jet., Comment: Published in ApJ Letters

The afterglow of GRB$\,$170817A/GW$\,$170817 was very unusual, slowly rising as $F_\nu\propto{}t_{\rm{}obs}^{0.8}\nu^{-0.6}$, peaking at $t_{\rm{obs,pk}}\sim\,150\;$days, and sharply decaying as $\sim{}t_{\rm{}obs}^{-2.2}$. VLBI observations revealed an unresolved radio afterglow image whose flux centroid moved superluminally with $v_{\rm{app}}\approx4c$, clearly indicating that the afterglow was dominated by a relativistic jet's compact core. Different jet angular structures explained the afterglow lightcurves: Gaussian and steep power-law profiles with narrow core angles $\theta_c\lesssim5^\circ$ and larger viewing angles $\theta_{\rm{}obs}/\theta_c\sim3-5$. However, a top-hat jet (with sharp edges at $\theta=\theta_0$) was ruled out since it appeared to produce an early flux rise much steeper than observed. Using 2D relativistic hydrodynamic simulations we show that the initial steep flux rise is an artifact caused by the simulation's finite start time, $t_0$, missing its flux contributions from $t

The main hard pulse of prompt gamma-ray emission in GRB$\,$170817A had a duration of $\sim0.5\,{\rm s}$ and its onset was delayed with respect to the gravitational-wave chirp signal by $t_{\rm del} \approx 1.74\,{\rm s}$. Detailed follow-up of the subsequent broadband kilonova emission revealed a two-component ejecta -- a lanthanide-poor ejecta with mass $M_{\rm ej,blue}\approx0.025\,M_\odot$ that powered the early but rapidly fading blue emission and a lanthanide-rich ejecta with mass $M_{\rm ej,red}\approx 0.04\,M_\odot$ that powered the longer lasting redder emission. Both the prompt gamma-ray onset delay and the existence of the blue ejecta with modest electron fraction, $0.2\lesssim Y_e\lesssim0.3$, can be explained if the collapse to a black hole was delayed by the formation of a hypermassive neutron star (HMNS). Here, we determine the survival time of the merger remnant by combining two different constraints, namely, the time needed to produce the requisite blue-ejecta mass and that necessary for the relativistic jet to bore its way out of the expanding ejecta. In this way, we determine that the remnant of GW170817 must have collapsed to a black hole after $t_{\rm coll}=0.98_{-0.26}^{+0.31}\,{\rm s}$. We also discuss how future detections and the delays between the gravitational and electromagnetic emissions can be used to constrain the properties of the merged object., Comment: 20 pages, 7 figures. Incorporated comments from the referee. Accepted for publication in ApJ

Gamma-ray burst afterglows arise from relativistic collisionless shocks in which the postshock tangled magnetic field $\vec{B}$ is produced by the two-stream and/or Weibel instabilities on plasma skin-depth scales $(c/\omega_p)$. The field is expected to be oriented predominantly within the shock plane ($B_{\perp}$; transverse to the shock normal, $\hat{n}_{\rm{sh}}$), and is often approximated to be completely within it ($B_\parallel\equiv\hat{n}_{\rm{sh}}\,\cdot\,\vec{B}=0$). Current 2D/3D particle-in-cell simulations are limited to short timescales and box sizes $\lesssim10^4(c/\omega_p)\ll R/\Gamma_{\rm{sh}}$ much smaller than the shocked region's comoving width, and cannot probe the asymptotic downstream $\vec{B}$ structure. We constrain the latter using the linear polarization upper limit, $|\Pi|<12\%$, on the radio afterglow of GW170817/GRB170817A. Afterglow polarization depends on the jet's angular structure, our viewing angle, and the $\vec{B}$ structure. In GW170817/GRB170817A the latter can be tightly constrained since the former two are constrained from observations. We model $\vec{B}$ as an isotropic field in 3D that is stretched along $\hat{n}_{\rm{sh}}$ by a factor $\xi\equiv{}B_{\parallel}/B_{\perp}$, whose initial value $\xi_f\equiv{}B_{\parallel,f}/B_{\perp,f}$ describes the field that survives downstream on plasma scales $\ll{}R/\Gamma_{\rm{sh}}$. We calculate $\Pi(\xi_f)$ by integrating over the shocked volume for core-dominated structured jets, with a local Blandford-McKee self-similar radial profile. We find that independent of the exact jet structure, $\vec{B}$ has a finite, but initially sub-dominant, parallel component: $0.57\lesssim\xi_f\lesssim0.89$, making it less anisotropic. While this motivates numerical studies of the asymptotic $\vec{B}$ structure in relativistic collisionless shocks, it may be consistent with turbulence amplified magnetic field., Comment: 11 pages, 6 figures, Accepted to MNRAS

In this White Paper we present the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related to Observatory Science targets. These include flaring stars, supernova remnants, accreting white dwarfs, low and high mass X-ray binaries, radio quiet and radio loud active galactic nuclei, tidal disruption events, and gamma-ray bursts. eXTP will be excellently suited to study one common aspect of these objects: their often transient nature. Developed by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Science, the eXTP mission is expected to be launched in the mid 2020s.

Despite being hard to measure, GRB prompt $\gamma$-ray emission polarization is a valuable probe of the dominant emission mechanism and the outflow's composition and angular structure. During the prompt emission the outflow is ultra-relativistic with Lorentz factors $\Gamma\gg1$. We describe in detail the linear polarization properties of various emission mechanisms: synchrotron radiation from different magnetic field structures (ordered: toroidal $B_{\rm tor}$ or radial $B_\parallel$, and random: normal to the radial direction $B_\perp$), Compton drag, and photospheric emission. We calculate the polarization for different GRB jet angular structures (e.g. top-hat, Gaussian, power-law) and viewing angles $\theta_{\rm obs}$. Synchrotron with $B_\perp$ can produce large polarizations, up to $25\%\lesssim\Pi\lesssim45\%$, for a top-hat jet but only for lines of sight just outside the jet's sharp edge. The same also holds for Compton drag, albeit with a slightly higher overall $\Pi$. Moreover, we demonstrate how $\Gamma$-variations during the GRB or smoother jet edges would significantly reduce $\Pi$. We construct a semi-analytic model for non-dissipative photospheric emission from structured jets. Such emission can produce up to $\Pi\lesssim15\%$ with reasonably high fluences, but this requires steep gradients in $\Gamma(\theta)$. A polarization of $50\%\lesssim\Pi\lesssim65\%$ can robustly be produced only by synchrotron emission from a transverse magnetic field ordered on angles $\gtrsim\!1/\Gamma$ around our line of sight (like a global toroidal field). Therefore, such a model would be strongly favored even by a single secure measurement within this range. We find that such a model would also be favored if $\Pi\gtrsim20\%$ is measured in most GRBs within a large enough sample, by deriving the polarization distribution for our different emission and jet models., Comment: 31 pages, 18 figures. Minor changes to the text; Accepted for publication in MNRAS

In this White Paper we present the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related to Observatory Science targets. These include flaring stars, supernova remnants, accreting white dwarfs, low and high mass X-ray binaries, radio quiet and radio loud active galactic nuclei, tidal disruption events, and gamma-ray bursts. eXTP will be excellently suited to study one common aspect of these objects: their often transient nature. Developed by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Science, the eXTP mission is expected to be launched in the mid 2020s., Comment: Accepted for publication on Sci. China Phys. Mech. Astron. (2019)

The X-ray to radio afterglow emission of GRB 170817A/GW 170817 so far scales as $F_\nu\propto\nu^{-0.6}t^{0.8}$ with observed frequency and time, consistent with a single power-law segment of the synchrotron spectrum from the external shock going into the ambient medium. This requires the effective isotropic equivalent afterglow shock energy in the visible region to increase as $\sim t^{1.7}$. The two main channels for such an energy increase are (i) \emph{radial}: more energy carried by slower material (in the visible region) gradually catches up with the afterglow shock and energizes it, and (ii) \emph{angular}: more energy in relativistic outflow moving at different angles to our line of sight, whose radiation is initially beamed away from us but its beaming cone gradually reaches our line of sight as it decelerates. One cannot distinguish between these explanations (or combinations of them) using only the X-ray to radio $F_\nu(t)$. Here we demonstrate that the most promising way to break this degeneracy is through afterglow imaging and polarization, by calculating the predicted evolution of the afterglow image (its size, shape and flux centroid) and linear polarization $\Pi(t)$ for different angular and/or radial outflow structures that fit $F_\nu(t)$. We consider two angular profiles -- a Gaussian and a narrow core with power-law wings in energy per solid angle, as well as a (cocoon motivated) (quasi-) spherical flow with radial velocity profile. For a jet viewed off-axis (and a magnetic field produced in the afterglow shock) $\Pi(t)$ peaks when the jet's core becomes visible, at $\approx2t_p$ where the lightcurve peaks at $t_p$, and the image can be elongated with aspect ratios$\;\gtrsim2$. A quasi-spherical flow has an almost circular image and a much lower $\Pi(t)$ (peaking at $\approx t_p$) and flux centroid displacement., Comment: 14 pages, 10 figures, Accepted for publication in MNRAS

In this White Paper we present the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related to Observatory Science targets. These include flaring stars, supernova remnants, accreting white dwarfs, low and high mass X-ray binaries, radio quiet and radio loud active galactic nuclei, tidal disruption events, and gamma-ray bursts. eXTP will be excellently suited to study one common aspect of these objects: their often transient nature. Developed by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Science, the eXTP mission is expected to be launched in the mid 2020s., Comment: Accepted for publication on Sci. China Phys. Mech. Astron. (2019)

Despite being hard to measure, GRB prompt $\gamma$-ray emission polarization is a valuable probe of the dominant emission mechanism and the outflow's composition and angular structure. During the prompt emission the outflow is ultra-relativistic with Lorentz factors $\Gamma\gg1$. We describe in detail the linear polarization properties of various emission mechanisms: synchrotron radiation from different magnetic field structures (ordered: toroidal $B_{\rm tor}$ or radial $B_\parallel$, and random: normal to the radial direction $B_\perp$), Compton drag, and photospheric emission. We calculate the polarization for different GRB jet angular structures (e.g. top-hat, Gaussian, power-law) and viewing angles $\theta_{\rm obs}$. Synchrotron with $B_\perp$ can produce large polarizations, up to $25\%\lesssim\Pi\lesssim45\%$, for a top-hat jet but only for lines of sight just outside the jet's sharp edge. The same also holds for Compton drag, albeit with a slightly higher overall $\Pi$. Moreover, we demonstrate how $\Gamma$-variations during the GRB or smoother jet edges would significantly reduce $\Pi$. We construct a semi-analytic model for non-dissipative photospheric emission from structured jets. Such emission can produce up to $\Pi\lesssim15\%$ with reasonably high fluences, but this requires steep gradients in $\Gamma(\theta)$. A polarization of $50\%\lesssim\Pi\lesssim65\%$ can robustly be produced only by synchrotron emission from a transverse magnetic field ordered on angles $\gtrsim\!1/\Gamma$ around our line of sight (like a global toroidal field). Therefore, such a model would be strongly favored even by a single secure measurement within this range. We find that such a model would also be favored if $\Pi\gtrsim20\%$ is measured in most GRBs within a large enough sample, by deriving the polarization distribution for our different emission and jet models., Comment: 31 pages, 18 figures. Minor changes to the text; Accepted for publication in MNRAS

The X-ray to radio afterglow emission of GRB 170817A/GW 170817 so far scales as $F_\nu\propto\nu^{-0.6}t^{0.8}$ with observed frequency and time, consistent with a single power-law segment of the synchrotron spectrum from the external shock going into the ambient medium. This requires the effective isotropic equivalent afterglow shock energy in the visible region to increase as $\sim t^{1.7}$. The two main channels for such an energy increase are (i) \emph{radial}: more energy carried by slower material (in the visible region) gradually catches up with the afterglow shock and energizes it, and (ii) \emph{angular}: more energy in relativistic outflow moving at different angles to our line of sight, whose radiation is initially beamed away from us but its beaming cone gradually reaches our line of sight as it decelerates. One cannot distinguish between these explanations (or combinations of them) using only the X-ray to radio $F_\nu(t)$. Here we demonstrate that the most promising way to break this degeneracy is through afterglow imaging and polarization, by calculating the predicted evolution of the afterglow image (its size, shape and flux centroid) and linear polarization $\Pi(t)$ for different angular and/or radial outflow structures that fit $F_\nu(t)$. We consider two angular profiles -- a Gaussian and a narrow core with power-law wings in energy per solid angle, as well as a (cocoon motivated) (quasi-) spherical flow with radial velocity profile. For a jet viewed off-axis (and a magnetic field produced in the afterglow shock) $\Pi(t)$ peaks when the jet's core becomes visible, at $\approx2t_p$ where the lightcurve peaks at $t_p$, and the image can be elongated with aspect ratios$\;\gtrsim2$. A quasi-spherical flow has an almost circular image and a much lower $\Pi(t)$ (peaking at $\approx t_p$) and flux centroid displacement., Comment: 14 pages, 10 figures, Accepted for publication in MNRAS

The short-duration ($\lesssim2\;$s) GRB 170817A in the nearby ($D=40\;$Mpc) elliptical galaxy NGC 4993 is the first electromagnetic counterpart of the first gravitational wave (GW) detection of a binary neutron-star (NS-NS) merger. It was followed by optical, IR, and UV emission from half a day up to weeks after the event, as well as late time X-ray and radio emission. The early UV, optical, and IR emission showed a quasi-thermal spectrum suggestive of radioactive-decay powered kilonova-like emission. Comparison to kilonova models favors the formation of a short-lived ($\sim1\;$s) hypermassive NS, which is also supported by the $\Delta t\approx1.74\;$s delay between the GW chirp signal and the prompt GRB onset. However, the late onset of the X-ray (8.9$\;$days) and radio (16.4$\;$days) emission, together with the low isotropic equivalent $\gamma$-ray energy output ($E_{\rm\gamma,iso}\approx5\times10^{46}\;$erg), strongly suggest emission from a narrow relativistic jet viewed off-axis. Here we set up a general framework for off-axis GRB jet afterglow emission, comparing analytic and numerical approaches, and showing their general predictions for short-hard GRBs that accompany binary NS mergers. The prompt GRB emission suggests a viewing angle well outside the jet's core, and we compare the afterglow lightcurves expected in such a case to the X-ray to radio emission from GRB 170817A. We fit an afterglow off-axis jet model to the X-ray and radio data and find that the observations are explained by a viewing angle $\theta_{\rm obs}\approx16^\circ-26^\circ$, GRB jet energy $E\sim10^{48.5}-10^{49.5}~{\rm erg}$, and external density $n\sim10^{-5}-10^{-1}~{\rm cm}^{-3}$ for a $\xi_e\sim 0.1$ non-thermal electron acceleration efficiency., Comment: 14 pages, 10 figures; Minor changes made to text; Fig. 10 now shows results from least-squares fit instead of MCMC simulation; updated references; Submitted to MNRAS

The first, long awaited, detection of a gravitational wave (GW) signal from the merger of a binary neutron-star (NS-NS) system was finally achieved (GW$\,$170817), and was also accompanied by an electromagnetic counterpart -- the short-duration GRB 170817A. It occurred in the nearby ($D\approx40\;$Mpc) elliptical galaxy NGC$\,$4993, and showed optical, IR and UV emission from half a day up to weeks after the event, as well as late time X-ray (at $\geq 8.9\;$days) and radio (at $\geq 16.4\;$days) emission. There was a delay of $\Delta t \approx 1.74\;$s between the GW merger chirp signal and the prompt-GRB emission onset, and an upper limit of $\theta_{\rm obs}<28^\circ$ was set on the viewing angle w.r.t the jet's symmetry axis from the GW signal. In this letter we examine some of the implications of these groundbreaking observations. The delay $\Delta t$ sets an upper limit on the prompt-GRB emission radius, $R_\gamma\lesssim 2c\Delta t/(\theta_{\rm obs}-\theta_0)^2$, for a jet with sharp edges at an angle $\theta_0<\theta_{\rm obs}$. GRB 170817A's relatively low isotropic equivalent $\gamma$-ray energy-output may suggest a viewing angle slightly outside the jet's sharp edge, $\theta_{\rm{}obs}-\theta_0\sim(0.05-0.1)(\Gamma/100)^{-1}$, but its peak $\nu{}F_\nu$ photon energy and afterglow emission suggest instead that the jet does not have sharp edges and the prompt emission was dominated by less energetic material along our line of sight, at $\theta_{\rm{}obs}\gtrsim 2\theta_0$. Finally, we consider the type of remnant that is produced by the NS-NS merger and find that a relatively long-lived ($>2\;$s) massive NS is strongly disfavored, while a hyper-massive NS of lifetime $\sim1\;$s appears to be somewhat favored over the direct formation of a black hole., Comment: 5 pages, 3 figures, minor changes; accepted to ApJL

The highly luminous and variable prompt emission in Gamma-Ray Bursts (GRBs) arises in an ultra-relativistic outflow. The exact underlying radiative mechanism shaping its non-thermal spectrum is still uncertain, making it hard to determine the outflow's bulk Lorentz factor $\Gamma$. GRBs with spectral cutoff due to pair production ($\gamma\gamma\to e^+e^-$) at energies $E_c\gtrsim10\;$MeV are extremely useful for inferring $\Gamma$. We find that when the emission region has a high enough compactness, then as it becomes optically thick to scattering, Compton downscattering by non-relativistic $e^\pm$-pairs can shift the spectral cutoff energy well below the self-annihilation threshold, $E_{\rm sa}=\Gamma m_ec^2/(1+z)$. We treat this effect numerically and show that $\Gamma$ obtained assuming $E_c=E_{\rm sa}$ can under-predict its true value by as much as an order of magnitude., Comment: 5 pages, 3 figures, Accepted for publication in MNRAS Letters

We study the linear and non-linear development of the Kruskal-Schwarzchild Instability in a relativisitically expanding striped wind. This instability is the generalization of Rayleigh-Taylor instability in the presence of a magnetic field. It has been suggested to produce a self-sustained acceleration mechanism in strongly magnetized outflows found in active galactic nuclei, gamma-ray bursts, and micro-quasars. The instability leads to magnetic reconnection, but in contrast with steady-state Sweet-Parker reconnection, the dissipation rate is not limited by the current layer's small aspect ratio. We performed two-dimensional (2D) relativistic magneto-hydrodynamic (RMHD) simulations featuring two cold and highly magnetized ($1\leq\sigma\leq10^{3}$) plasma layers with an anti-parallel magnetic field separated by a thin layer of relativistically hot plasma with a local effective gravity induced by the outflow's acceleration. Our simulations show how the heavier relativistically hot plasma in the reconnecting layer drips out and allows oppositely oriented magnetic field lines to reconnect. The instability's growth rate in the linear regime matches the predictions of linear stability analysis. We find turbulence rather than an ordered bulk flow near the reconnection region, with turbulent velocities up to $\sim0.1$c, largely independent of model parameters. However, the magnetic energy dissipation rate is found to be much slower, corresponding to an effective ordered bulk velocity inflow into the reconnection region $v_{\rm in}=\beta_{\rm in}c$, of $10^{-3}\lesssim\beta_{\rm in}\lesssim 5\times10^{-3}$. This occurs due to the slow evacuation of hot plasma from the current layer, largely because of the Kelvin-Helmholtz instability experienced by the dripping plasma. 3D RMHD simulations are needed to further investigate the non-linear regime., Comment: 12 pages, 12 figures, Accepted for Publication in MNRAS

We analyzed broad-band X-ray and radio data of the magnetar SGR J1935+2154 taken in the aftermath of its 2014, 2015, and 2016 outbursts. The source soft X-ray spectrum <10 keV is well described with a BB+PL or 2BB model during all three outbursts. NuSTAR observations revealed a hard X-ray tail, $\Gamma=0.9$, extending up to 79 keV, with flux larger than the one detected <10 keV. Imaging analysis of Chandra data did not reveal small-scale extended emission around the source. Following the outbursts, the total 0.5-10 keV flux from SGR J1935+2154 increased in concordance to its bursting activity, with the flux at activation onset increasing by a factor of $\sim7$ following its strongest June 2016 outburst. A Swift/XRT observation taken 1.5 days prior to the onset of this outburst showed a flux level consistent with quiescence. We show that the flux increase is due to the PL or hot BB component, which increased by a factor of $25$ compared to quiescence, while the cold BB component $kT=0.47$ keV remained more or less constant. The 2014 and 2015 outbursts decayed quasi-exponentially with time-scales of $\sim40$ days, while the stronger May and June 2016 outbursts showed a quick short-term decay with time-scales of $\sim4$ days. Our Arecibo radio observations set the deepest limits on the radio emission from a magnetar, with a maximum flux density limit of 14 $\mu$Jy for the 4.6 GHz observations and 7 $\mu$Jy for the 1.4 GHz observations. We discuss these results in the framework of the current magnetar theoretical models., Comment: 17 pages, 7 figures, 5 tables, submitted to ApJ, comments welcome

The highly luminous and variable prompt emission in Gamma-Ray Bursts (GRBs) arises in an ultra-relativistic outflow. The exact underlying radiative mechanism shaping its non-thermal spectrum is still uncertain, making it hard to determine the outflow's bulk Lorentz factor $\Gamma$. GRBs with spectral cutoff due to pair production ($\gamma\gamma\to e^+e^-$) at energies $E_c\gtrsim10\;$MeV are extremely useful for inferring $\Gamma$. We find that when the emission region has a high enough compactness, then as it becomes optically thick to scattering, Compton downscattering by non-relativistic $e^\pm$-pairs can shift the spectral cutoff energy well below the self-annihilation threshold, $E_{\rm sa}=\Gamma m_ec^2/(1+z)$. We treat this effect numerically and show that $\Gamma$ obtained assuming $E_c=E_{\rm sa}$ can under-predict its true value by as much as an order of magnitude., Comment: 5 pages, 3 figures, Accepted for publication in MNRAS Letters

The short-duration ($\lesssim2\;$s) GRB 170817A in the nearby ($D=40\;$Mpc) elliptical galaxy NGC 4993 is the first electromagnetic counterpart of the first gravitational wave (GW) detection of a binary neutron-star (NS-NS) merger. It was followed by optical, IR, and UV emission from half a day up to weeks after the event, as well as late time X-ray and radio emission. The early UV, optical, and IR emission showed a quasi-thermal spectrum suggestive of radioactive-decay powered kilonova-like emission. Comparison to kilonova models favors the formation of a short-lived ($\sim1\;$s) hypermassive NS, which is also supported by the $\Delta t\approx1.74\;$s delay between the GW chirp signal and the prompt GRB onset. However, the late onset of the X-ray (8.9$\;$days) and radio (16.4$\;$days) emission, together with the low isotropic equivalent $\gamma$-ray energy output ($E_{\rm\gamma,iso}\approx5\times10^{46}\;$erg), strongly suggest emission from a narrow relativistic jet viewed off-axis. Here we set up a general framework for off-axis GRB jet afterglow emission, comparing analytic and numerical approaches, and showing their general predictions for short-hard GRBs that accompany binary NS mergers. The prompt GRB emission suggests a viewing angle well outside the jet's core, and we compare the afterglow lightcurves expected in such a case to the X-ray to radio emission from GRB 170817A. We fit an afterglow off-axis jet model to the X-ray and radio data and find that the observations are explained by a viewing angle $\theta_{\rm obs}\approx16^\circ-26^\circ$, GRB jet energy $E\sim10^{48.5}-10^{49.5}~{\rm erg}$, and external density $n\sim10^{-5}-10^{-1}~{\rm cm}^{-3}$ for a $\xi_e\sim 0.1$ non-thermal electron acceleration efficiency., Comment: 14 pages, 10 figures; Minor changes made to text; Fig. 10 now shows results from least-squares fit instead of MCMC simulation; updated references; Submitted to MNRAS

The first, long awaited, detection of a gravitational wave (GW) signal from the merger of a binary neutron-star (NS-NS) system was finally achieved (GW$\,$170817), and was also accompanied by an electromagnetic counterpart -- the short-duration GRB 170817A. It occurred in the nearby ($D\approx40\;$Mpc) elliptical galaxy NGC$\,$4993, and showed optical, IR and UV emission from half a day up to weeks after the event, as well as late time X-ray (at $\geq 8.9\;$days) and radio (at $\geq 16.4\;$days) emission. There was a delay of $\Delta t \approx 1.74\;$s between the GW merger chirp signal and the prompt-GRB emission onset, and an upper limit of $\theta_{\rm obs}<28^\circ$ was set on the viewing angle w.r.t the jet's symmetry axis from the GW signal. In this letter we examine some of the implications of these groundbreaking observations. The delay $\Delta t$ sets an upper limit on the prompt-GRB emission radius, $R_\gamma\lesssim 2c\Delta t/(\theta_{\rm obs}-\theta_0)^2$, for a jet with sharp edges at an angle $\theta_0<\theta_{\rm obs}$. GRB 170817A's relatively low isotropic equivalent $\gamma$-ray energy-output may suggest a viewing angle slightly outside the jet's sharp edge, $\theta_{\rm{}obs}-\theta_0\sim(0.05-0.1)(\Gamma/100)^{-1}$, but its peak $\nu{}F_\nu$ photon energy and afterglow emission suggest instead that the jet does not have sharp edges and the prompt emission was dominated by less energetic material along our line of sight, at $\theta_{\rm{}obs}\gtrsim 2\theta_0$. Finally, we consider the type of remnant that is produced by the NS-NS merger and find that a relatively long-lived ($>2\;$s) massive NS is strongly disfavored, while a hyper-massive NS of lifetime $\sim1\;$s appears to be somewhat favored over the direct formation of a black hole., Comment: 5 pages, 3 figures, minor changes; accepted to ApJL

We study the linear and non-linear development of the Kruskal-Schwarzchild Instability in a relativisitically expanding striped wind. This instability is the generalization of Rayleigh-Taylor instability in the presence of a magnetic field. It has been suggested to produce a self-sustained acceleration mechanism in strongly magnetized outflows found in active galactic nuclei, gamma-ray bursts, and micro-quasars. The instability leads to magnetic reconnection, but in contrast with steady-state Sweet-Parker reconnection, the dissipation rate is not limited by the current layer's small aspect ratio. We performed two-dimensional (2D) relativistic magneto-hydrodynamic (RMHD) simulations featuring two cold and highly magnetized ($1\leq\sigma\leq10^{3}$) plasma layers with an anti-parallel magnetic field separated by a thin layer of relativistically hot plasma with a local effective gravity induced by the outflow's acceleration. Our simulations show how the heavier relativistically hot plasma in the reconnecting layer drips out and allows oppositely oriented magnetic field lines to reconnect. The instability's growth rate in the linear regime matches the predictions of linear stability analysis. We find turbulence rather than an ordered bulk flow near the reconnection region, with turbulent velocities up to $\sim0.1$c, largely independent of model parameters. However, the magnetic energy dissipation rate is found to be much slower, corresponding to an effective ordered bulk velocity inflow into the reconnection region $v_{\rm in}=\beta_{\rm in}c$, of $10^{-3}\lesssim\beta_{\rm in}\lesssim 5\times10^{-3}$. This occurs due to the slow evacuation of hot plasma from the current layer, largely because of the Kelvin-Helmholtz instability experienced by the dripping plasma. 3D RMHD simulations are needed to further investigate the non-linear regime., Comment: 12 pages, 12 figures, Accepted for Publication in MNRAS

We analyzed broad-band X-ray and radio data of the magnetar SGR J1935+2154 taken in the aftermath of its 2014, 2015, and 2016 outbursts. The source soft X-ray spectrum <10 keV is well described with a BB+PL or 2BB model during all three outbursts. NuSTAR observations revealed a hard X-ray tail, $\Gamma=0.9$, extending up to 79 keV, with flux larger than the one detected <10 keV. Imaging analysis of Chandra data did not reveal small-scale extended emission around the source. Following the outbursts, the total 0.5-10 keV flux from SGR J1935+2154 increased in concordance to its bursting activity, with the flux at activation onset increasing by a factor of $\sim7$ following its strongest June 2016 outburst. A Swift/XRT observation taken 1.5 days prior to the onset of this outburst showed a flux level consistent with quiescence. We show that the flux increase is due to the PL or hot BB component, which increased by a factor of $25$ compared to quiescence, while the cold BB component $kT=0.47$ keV remained more or less constant. The 2014 and 2015 outbursts decayed quasi-exponentially with time-scales of $\sim40$ days, while the stronger May and June 2016 outbursts showed a quick short-term decay with time-scales of $\sim4$ days. Our Arecibo radio observations set the deepest limits on the radio emission from a magnetar, with a maximum flux density limit of 14 $\mu$Jy for the 4.6 GHz observations and 7 $\mu$Jy for the 1.4 GHz observations. We discuss these results in the framework of the current magnetar theoretical models., Comment: 17 pages, 7 figures, 5 tables, submitted to ApJ, comments welcome

We have compiled the most comprehensive burst sample from magnetar 4U 0142+61, comprising 27 bursts from its three burst-active episodes in 2011, 2012 and the latest one in 2015 observed with Swift/BAT and Fermi/GBM. Bursts from 4U 0142+61 morphologically resemble typical short bursts from other magnetars. However, 4U 0142+61 bursts are less energetic compared to the bulk of magnetar bursts. We uncovered an extended tail emission following a burst on 2015 February 28, with a thermal nature, cooling over a time-scale of several minutes. During this tail emission, we also uncovered pulse peak phase aligned X-ray bursts , which could originate from the same underlying mechanism as that of the extended burst tail, or an associated and spatially coincident but different mechanism., Comment: Accepted for publication in ApJ

The first wind nebula around a magnetar was recently discovered in X-rays around Swift~J1834.9$-$0846. We study this magnetar's global energetics and the properties of its particle wind or outflows. At a distance of $\sim4\;$kpc, Swift~J1834.9$-$0846 is located at the center of the supernova remnant (SNR) W41 whose radius is $\sim 19\;$pc, an order of magnitude larger than that of the X-ray nebula ($\sim2\;$pc). The association with SNR W41 suggests a common age of $\sim5-100\;$kyr, while its spin-down age is $4.9$~kyr. A small natal kick velocity may partly explain why a wind nebula was detected around this magnetar but not around other magnetars, most of which appear to have larger kick velocities and may have exited their birth SNR. We find that the GeV and TeV source detected by Fermi/LAT and H.E.S.S., respectively, of radius $\sim11\;$pc is most likely of hadronic origin. The dynamics and internal structure of the nebula are examined analytically to explain the nebula's current properties. Its size may naturally correspond to the diffusion-dominated cooling length of the X-ray emitting $e^+e^-$ pairs. This may also account for the spectral softening of the X-ray emission from the nebula's inner to outer parts. Analysis of the X-ray synchrotron nebula implies that (i) the nebular magnetic field is $\gtrsim 11\;\mu$G (and likely $\lesssim30\;\mu$G), and (ii) the nebula is not powered predominantly by the magnetar's quiescent spin-down-powered MHD wind, but by other outflows that contribute most of its energy. The latter are most likely associated with the magnetar's bursting activity, and possibly dominated by outflows associated with its past giant flares. The energy source for the required outflows cannot be the decay of the magnetar's dipole field alone, and is most likely the decay of its much stronger internal magnetic field., Comment: 33 pages, 15 figures. Accepted for publication in MNRAS

We have compiled the most comprehensive burst sample from magnetar 4U 0142+61, comprising 27 bursts from its three burst-active episodes in 2011, 2012 and the latest one in 2015 observed with Swift/BAT and Fermi/GBM. Bursts from 4U 0142+61 morphologically resemble typical short bursts from other magnetars. However, 4U 0142+61 bursts are less energetic compared to the bulk of magnetar bursts. We uncovered an extended tail emission following a burst on 2015 February 28, with a thermal nature, cooling over a time-scale of several minutes. During this tail emission, we also uncovered pulse peak phase aligned X-ray bursts , which could originate from the same underlying mechanism as that of the extended burst tail, or an associated and spatially coincident but different mechanism., Comment: Accepted for publication in ApJ

The first wind nebula around a magnetar was recently discovered in X-rays around Swift~J1834.9$-$0846. We study this magnetar's global energetics and the properties of its particle wind or outflows. At a distance of $\sim4\;$kpc, Swift~J1834.9$-$0846 is located at the center of the supernova remnant (SNR) W41 whose radius is $\sim 19\;$pc, an order of magnitude larger than that of the X-ray nebula ($\sim2\;$pc). The association with SNR W41 suggests a common age of $\sim5-100\;$kyr, while its spin-down age is $4.9$~kyr. A small natal kick velocity may partly explain why a wind nebula was detected around this magnetar but not around other magnetars, most of which appear to have larger kick velocities and may have exited their birth SNR. We find that the GeV and TeV source detected by Fermi/LAT and H.E.S.S., respectively, of radius $\sim11\;$pc is most likely of hadronic origin. The dynamics and internal structure of the nebula are examined analytically to explain the nebula's current properties. Its size may naturally correspond to the diffusion-dominated cooling length of the X-ray emitting $e^+e^-$ pairs. This may also account for the spectral softening of the X-ray emission from the nebula's inner to outer parts. Analysis of the X-ray synchrotron nebula implies that (i) the nebular magnetic field is $\gtrsim 11\;\mu$G (and likely $\lesssim30\;\mu$G), and (ii) the nebula is not powered predominantly by the magnetar's quiescent spin-down-powered MHD wind, but by other outflows that contribute most of its energy. The latter are most likely associated with the magnetar's bursting activity, and possibly dominated by outflows associated with its past giant flares. The energy source for the required outflows cannot be the decay of the magnetar's dipole field alone, and is most likely the decay of its much stronger internal magnetic field., Comment: 33 pages, 15 figures. Accepted for publication in MNRAS

Gamma-ray bursts (GRBs) show a dramatic pulse structure that requires bulk relativistic motion, but whose physical origin has remained murky. We focus on a hot, magnetized jet that is emitted by a black hole and interacts with a confining medium. Strongly relativistic expansion of the magnetic field, as limited by a corrugation instability, may commence only after it forms a thin shell. Then the observed $T_{90}$ burst duration is dominated by the curvature delay, and null periods arise from angular inhomogeneities, not the duty cycle of the engine. We associate the $O(1)$ s timescale observed in the pulse width distribution of long GRBs with the collapse of the central 2.5-3$M_\odot$ of a massive stellar core. A fraction of the baryons are shown to be embedded in the magnetized outflow by the hyper-Eddington radiation flux; they strongly disturb the magnetic field after the compactness drops below $\sim 4\times 10^3(Y_e/0.5)^{-1}$. The high-energy photons so created have a compressed pulse structure. Delayed breakout of magnetic field from heavier baryon shells is also a promising approach to X-ray flares. In the second part of the paper, we calculate the imprint of an expanding, scattering photosphere on pulse evolution. Two models for generating the high-energy spectral tail are contrasted: i) pair breakdown due to reheating of an optically thin pair plasma embedded in a thermal radiation field; and ii) continuous heating extending from large to small scattering depth. The second model is strongly inconsistent with the observed hard-to-soft evolution in GRB pulses. The first shows some quantitative differences if the emission is purely spherical, but we show that finite shell width, mild departures from spherical curvature, and latitudinal Lorentz factor gradients have interesting effects., Comment: Submitted to the ApJ, 21 pages, 22 figures

We consider delayed, volumetric heating in a magnetized outflow that has broken out of a confining medium and expanded to a high Lorentz factor ($\Gamma \sim 10^2-10^3$) and low optical depth to scattering ($\tau_{\rm T} \sim 10^{-3}-10^{-2}$). The energy flux at breakout is dominated by the magnetic field, with a modest contribution from quasi-thermal gamma rays whose spectrum was calculated in Paper I. We focus on the case of extreme baryon depletion in the magnetized material, but allow for a separate baryonic component that is entrained from a confining medium. Dissipation is driven by relativistic motion between these two components, which develops once the photon compactness drops below $ 4\times 10^3(Y_e/0.5)^{-1}$. We first calculate the acceleration of the magnetized component following breakout, showing that embedded MHD turbulence provides significant inertia, the neglect of which leads to unrealistically high estimates of flow Lorentz factor. After re-heating begins, the pair and photon distributions are evolved self-consistently using a one-zone kinetic code that incorporates an exact treatment of Compton scattering, pair production and annihilation, and Coulomb scattering. Heating leads to a surge in pair creation, and the scattering depth saturates at $\tau_{\rm T} \sim$ 1-4. The plasma maintains a very low ratio of particle to magnetic pressure, and can support strong anisotropy in the charged particle distribution, with cooling dominated by Compton scattering. High-energy power-law spectra with photon indices in the range observed in GRBs ($-3 < \beta < -3/2$) are obtained by varying the ratio of heat input to the seed energy in quasi-thermal photons. We contrast our results with those for continuous heating across an expanding photosphere, and show that the latter model produces soft-hard evolution that is inconsistent with observations of GRBs., Comment: Submitted to the ApJ, 27 pages, 19 figures